1. Field of the Invention
The present invention relates to the transferring of a signal in water or in oil and more particularly to a signal transmitting and receiving device using ultrasonic wave.
2. Description of the Prior Art
The present inventors have already suggested that the combination of a plurality of frequency signals and a plurality of unit time regions should be used for the transferring of information in a signal transmitting and receiving device using ultrasonic wave.
This information transferring system using the combination of frequencies and time regions is explained assuming that a frequency-time matrix (hereinafter referred to as FT matrix) is comprised by m different frequency signals f.sub.1, f.sub.2, f.sub.3, . . . f.sub.m in a row and n unit time regions t.sub.1, t.sub.2, t.sub.3, . . . t.sub.n in a column. When the m frequency signals are selected on the condition that the unit time region t.sub.1 always contains one frequency signal and the same frequency signal is not used in two or more of the n unit time regions, the total number of combination patterns of frequencies and unit time regions (hereinafter referred to as FT patterns) is expressed as follows. EQU M = m.sup.. (n - 1) ! /(n - m) ! (1)
In the transferring of information by the use of such FT patterns, it is common practice that the operators at the transmitting and receiving sides determine previously that each of the FT patterns represents a specific unit information piece such as an alphabetical character or a numerical value. The operator at the transmitting side resolves any given information into a plurality of unit information pieces and sequentially converts the unit information pieces into FT patterns in accordance with which frequency signals are sequentially oscillated in the respective unit time regions. The operator at the receiving side can know the contents of the information by decoding the frequency signals sequentially.
The above-described system has the advantage that each unit information piece is transferred within a shorter time as compared with other systems such as an FSK (Frequency Shift Keying) system, especially in the transferring of information underwater and that errors can be easily detected. This provides a possibility for the realization of communications with high speed and high reliability. Another advantage of the above-described system is the elimination of the inconvenience resulting from the fixing of FT patterns as is seen in RADAS (Random Access Discrete Address System) similar to the system under consideration. Incidentally, the inconvenience of the RADAS is that since different FT patterns are allotted to different receiving stations with which a transmitting station interchanges information, the transmitting station is required to transmit information including the allotted FT pattern to the associated receiving station. This limits the receiving stations to which the information is capable of being transmitted, with the result that it is impossible to transfer specific information to a plurality of receiving stations at the same time, thus restricting the field of applications of the system.